Coho salmon (Oncorhynchus kisutch) are more resistant than chinook salmon (O. tshawytscha) to experimental infection with the glochidia of the freshwater mussel Margaritifera margaritifera. Histological sections of gills from coho salmon 16 hr postinfection (p.i.) showed that parasite encystment either did not occur or had progressed incompletely, which accounted for the loss of many glochidia from the gills. The remaining encysted glochidia were sloughed within 2 days (p.i.) by a well-developed hyperplasia. On chinook salmon, the parasites developed normally with no sloughing or hyperplasia. Analysis of blood samples taken from coho salmon at intervals during the infection showed significant increases in hematocrit, hemoglobin, the mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and leukocyte numbers when compared with control fish. In infected chinook salmon only the hematocrit, erythrocyte numbers, and MCV increased while the MCHC decreased. Total plasma protein increased in coho salmon but decreased in chinook salmon during infection. Glochidial antibodies were demonstrated in the blood plasma of coho and chinook salmon 8 to 12 wk p.i. Fewer glochidia attached to the excised gills of coho salmon than to the gills of chinook salmon. Also, the in vitro survival time of parasites in mucus and plasma from coho salmon was less than in the same chinook salmon fluids.
